The 3D Manoeuvre Gear lets soldiers fly about and kill the human-devouring giants in Attack on Titan, but for actors Haruma Miura and Kiko Mizuhara, the bulky device was a lot of trouble.

"It's huge, and the scabbards stick out on both sides behind you," Mizuhara, 24, tells Young Post through a translator. "We couldn't sit down with the gear harnessed on."

Mizuhara, who plays Mikasa, had two months of intense gym training before filming to strengthen her core muscles and flexibility. She sometimes practised for up to eight hours a day. "The flight scenes weren't so bad. I think pulling off the action scenes on the ground was tougher," she says, as her co-star Miura, laughs beside her.

It was also the first time Miura used wires, but the biggest challenge for him was his character Eren's amateur fighting style. "Eren's not a natural fighter, so when I fought as him I had to make a lot of intentional mistakes," says the 25-year-old. "It was hard to make that look natural."

Chatting on Monday ahead of the premier of the sequel, Attack on Titan: End of the World, the two actors seem a goofy pair. While gaping at the panoramic view of Victoria Harbour from a luxury suite in a five-star hotel, they laugh loudly, gesture widely and stick out their tongues every now and then. Well-dressed, they couldn't be more different than their traumatised characters in the film.

Attack on Titan and its sequel are loosely based on the first three volumes of 29-year-old Hajime Isayama's popular manga series. The films depict a post-apocalyptic Japan where humans live within massive walls to protect them from Titans. When a 50-metre Titan smashes a hole in the wall for smaller Titans to enter, childhood friends Eren and Mikasa flee to a church. But Mikasa stays outside to rescue a baby, and is grabbed by a Titan, leaving Eren feeling guilty that he couldn't save her. Only after he becomes a soldier, and the pair meet again outside the wall, does he find out that Mikasa survived.

Mizuhara isn't afraid to get a little dirty for a role.

The film is an action-horror, but behind the scenes, there was a lot of comedy. "There's a part when Eren's face is pulled sideways by fibres after he transforms to a Titan," giggles Mizuhara. "It looks real enough in the film, but it was hilarious to see Miura with make-up, as if someone was pinching his face. We had fun laughing at him."

But Miura protests: "I thought I was very popular. Everyone was dying for a selfie with me."

The first film opened in Hong Kong last month, and though it topped the Japanese box-office, it was widely criticised for its sloppy production and poor acting. Many manga fans were also critical of the plot and changes to characters.

The filmmakers switched the set from a European location to Japan, and removed many key characters from the original story. As critics prepare to slam the sequel, too, Miura says the modified story still has its own appeal.

"Many boys might also fail to protect their loved ones in reality, so people would understand Eren's feeling in the film," he says.

"I think it's very relatable as well," adds Mizuhara. "Mikasa also transforms from a weak girl to someone who is very self-protective, before learning to help others again. Most girls look weak on the outside but they're actually very strong."

Mizuhara also thinks Mikasa's relationship with Captain Shikishima, the man who saved her from the Titan, could be more deeply explored. Maybe it deserves its own spin-off?

"Nah … I wouldn't want to watch that," teases Miura.

The producers have yet to decide if a third film will be made. "Let's see how this film does. If it's good enough, they might make a third one," says Mizuhara.

Attack on Titan: End of the World opens on Saturday, September 19.

Video of Attack on Titan (Part 2): END OF THE WORLD (Live Action Movie) - Official Theatrical Trailer